This invention relates to a method and apparatus for producing nuclear spin polarized xenon gas, and more specifically, relates to continuously producing highly concentrated nuclear spin polarized xenon gas capable of producing a polarized nuclear spin useful for NMR+MRI apparatus.
It has been recently reported that when xenon gas with a nuclear spin polarized (nuclear spin polarized xenon gas) is applied to NMR·MRI method, detection sensitivity is enhanced rapidly.
The term ‘polarized’ as used herein means that distribution of the spin number occupying energy order of nuclear spins of an atomic nucleus corresponding to the orientation state with respect to the metal static magnetic field is extremely one-sided.
To obtain a rare gas having the polarized state circularly polarized excitation light is irradiated on gas having rare gas including a single atomic molecule having a nuclear spin of spin quantum number 112 such as xenon-129 (129Xe), helium-3 (9He) or the like mixed with alkali metal vapor such as rubidium, cesium or the like. An electron in the base state order of rubidium or the like is excited by light absorption in the base state order and returned to the base state order after passage of the base state order, at which time it is transited with high probability to one order of electron orders out of the base state orders whose degeneracy is released magnetically by a magnetic field. The magnetic field is applied from outside to prepare the state that an electronic spin polarization degree of a rubidium molecule or the like is high, and the rubidium or the like in the high polarization state collides with rare gas such as xenon, at which channel the high polarization state of rubidium or the like moves to a nuclear spin system of rare gas such as xenon. This channel is generally called optical pumping.
In the conventional polarized rare gas producing apparatus, a mixed gas of rare gas and alkali metal vapor is confined into an optical reaction vessel, to which irradiation of excited light and application of magnetic field are carried out. For example, there is a producing apparatus, for the purpose of using polarized helium-3 of high density as a neutral polarizer, in which a mixed gas of helium-3 gas and nitrogen gas and alkali metal are confined into a cylindrical glass ample (for example, see M. E. Wagshul and T. E. Chu P P, Phy, Rev. A40, 4447 (1989)).
On the other hand, there is an apparatus in which for example, 1% of xenon is mixed with buffer gas of helium of 10 atmospheric pressure or so, introduced into a cylindrical glass vessel, irradiated, polarized, and guided into Dewar cooled by liquid nitrogen from a gas outlet of the vessel. Polarized xenon is formed into a solid, which is separated. The remaining helium gas is discharged from a vent line (For example, see B. Driehuys, G. D. Cates, E. Miron, K. Sauer, D. K. Walter and W. Happer, Appl. Phys, Lett. 69, 1668 (1996).
In any of those apparatuses as noted above, operation for enhancing the polarization rate is carried out by receiving a laser beam in the state that rare gas or the like is stayed in an optical reaction vessel. With the polarization rate enhanced, and after the temperature cooled to a room temperature, the gas may be used as a neutral polarizer as it is, or polarized as xenon 129 once solid-separated within Dewar is heated again into gas. Then it is transferred to a separate vessel for use in measurement of NMR or the like.
However, in the above-mentioned conventional method, for facilitating polarization, xenon is diluted, for example, to helium 2% xenon concentration or so. Polarized gas containing produced xenon then is frozen with liquid nitrogen, which is heated to remove only the xenon to produce high concentration xenon gas, thus posing a problem that work efficiency is extremely poor. In addition, in the conventional apparatus in which gas or the like is stayed and polarized, since polarized gas cannot be generated continuously, polarized gas is taken out into a separate vessel every time and carried to NMR apparatus, thus taking time, and posing a problem that the polarization rate reduces during such a period of time as described.
This invention has been accomplished in view of the foregoing, and has its object to provide producing method and producing apparatus capable of obtaining polarized xenon gas of high concentration without being frozen, and capable of generating polarized xenon gas continuously.
It is a further object of this invention to provide producing method and producing apparatus of a glass cell in which metal rubidium and xenon gas used for the above-described producing method and producing apparatus are solidified and sealed under the absence of oxygen.